1. Field of the Invention
This invention relates generally to voltage comparators and, more particularly, to a circuit for detecting when an input becomes greater than the supply voltage by predetermined offset voltage.
2. Description of the Prior Art
The advantages offered by MOS technology are well known; e.g. higher density, greater yield, etc. Thus, smaller MOS device geometries permit a greater number of devices to be produced per unit area or, stated another way, a single MOS device will occupy less space. This characteristic is extremely important in the design and fabrication of complex digital integrated circuits; for example, single chip microprocessors.
Whereas digital circuitry is generally characterized by its "ON/OFF" or "ONE/ZERO" nature, most measurements in the real world are inherently analog; e.g., temperature, pressure, speed, voltage, etc. Therefore, it is necessary that microprocessors and other digital circuitry communicate or interface with analog circuitry such as amplifiers, buffers, comparators, etc., in order to permit digital processing of the analog signals. The required interfacing may be accomplished by providing analog components which are external to the microprocessor chip. However, such arrangements generally require more current, a larger power supply and commonly present more opportunities for design and manufacturing errors. To avoid these disadvantages, analog circuits are being manufactured integrally with the digital circuitry; e.g., on the microprocessor chip itself, and due to the complex nature of microprocessors, the inclusion of analog devices on the same chip requires that the same manufacturing process be employed.
In some cases, it may be necessary to detect when the input voltage has exceeded the supply voltage in order to initiate certain circuit operations. For example, a microcomputer which normally functions with two voltage levels (zero and 5 volts) may include a feature whereby a self-test routine is initiated when the input voltage exceeds the supply voltage. However, this self-test feature should not be instigated until the input voltage has exceeded the supply voltage by some predetermined amount in order to avoid unwanted initiations close to the 5 volt level. That is, slight variations in the supply voltage should not be allowed to instigate the self-test feature.
Devices which detect equivalence of two voltages levels or the lack thereof are well-known. For example, a supply voltage and an input voltage could be applied to the inputs of a comparator which would then generate an output change when the two input voltages were equal. To avoid problems at the 5 volt level, an offset voltage could be added to the supply voltage; however, problems still arise due to the difficulty of maintaining the offset voltage constant. If the offset voltage should become too low, the trigger voltage could again reside in the 5 volt level.
Low ratio MOS inverters have also been used in attempt to achieve the desired results. However, it has been found that when using low ratio inverters, the switching point can vary from approximately 3 volts to approximately 20 volts, clearly an unsatisfactory solution.